"Blue Moon"

Have you ever heard the term “Once in a blue moon?”

This expression is commonly used metaphorically to describe the rarity of an event. However a true “Blue Moon” is the second full moon in a calendar month. This generally happens on average every 2 1/2 to 3 years.

Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month.

The next “Blue Moon” will occur this year in December.  The first full moon of that month will occur on December 2.  The second and “Blue Moon” will occur on December 31.